The main purpose of this investigation is to understand how 5-iodo-2'-deoxyuridine (IdUrd) induces alkaline phosphatase activity in the human cancer cell, HeLa. Studies are proposed in which (1) the biochemical properties of purified alkaline phosphatase from control and IdUrd treated cells are examined and compared, (2) nucleotide pools are measured following administration of IdUrd and deoxycytidine, and (3) the effects of various inhibitors of induction, principally xanthiae derivatives, are investigated in mutant, thioguanine-resistant HeLa cells for comparison in non-mutant HeLa cells.